1. Field of the Invention
The present invention relates to a display monitor having a DDC (Display Data Channel) function, and more particularly to an input channel switching control device for a display monitor which includes a DVI-I interface capable of inputting video signals of analog specification and digital specification.
2. Description of the Background Art
The DDC function is a function defined in the DDC standard, for achieving an automatic environment setting function, i.e., plug-and-play, on a computer system. The DDC standard defines signal lines and a procedure for data exchange between a computer and a display monitor. That is, a display monitor supporting the DDC standard is connected to a computer, the computer can take information on the display monitor (hereinafter, referred to as “EDID (Extended Display IDentification)”), which is required for the plug-and-play, from the display monitor through a DDC communication with the display monitor.
The EDID data is stored in a nonvolatile memory such as an EEPROM (Electrically Erasable and Programmable Read Only Memory) provided inside the display monitor and contains information on manufacturer/product ID, specification on the display monitor, support timing and the like.
Further, an analog interface including a typical D-Sub connector specification has been conventionally used as an interface of a display monitor. Most of flat displays such as a liquid crystal display (LCD), however, have a digital interface, and for example, when a display such as an LCD having the digital interface is connected to a computer having an output of D-Sub connector specification, there is apprehension that deterioration of display quality and an increase in cost for the device are caused by analog/digital (A/D) conversion of video signals, and standardization of digital interface is highly expected.
The DVI (Digital Visual Interface) is a digital interface standardized by the DDWG (Digital Display Working Group), whose digital data transmission system consists of total four-channel transmission lines, three-channel image data transmission lines (red, green, blue) and one-channel clock transmission line. Serial signals of each transmission line adopt TMDS (Transmission minimized Differential Signaling) which is a system using single-end differential signals.
FIGS. 11 and 12 are views showing connectors of DVI specification. The DVI specification includes two types, an interface supporting signals of only digital specification as shown in FIG. 11 and that supporting signals of both digital specification and analog specification as shown in FIG. 12. The former is called DVI-D and the latter is called DVI-I. According to the DVI specification, two signal lines are used for the DDC communication, one is an SDA line which is a bidirectional serial data line and the other is an SCL line used as a clock. Further, in the DVI specification, a communications protocol called DDC2B is used for the DDC communication.
When a computer requests the display monitor to transmit the EDID data through the seventh pin (SDA) and the sixth pin (SCL) of the DVI connector, the display monitor begins to transmit the EDID data to the computer through the seventh pin (SDA). Then, when the computer judges that the EDID data is sufficiently transmitted, the computer requests the display monitor to stop transmission of the EDID data and the display monitor, receiving the request, stops transmission of the EDID data.
In the case of DVI-I interface, as shown in FIG. 12, the input interface for the video signals has two systems, i.e., a digital channel and an analog channel, while the DDC line has only one system. In the display monitor of DVI-I specification, therefore, it is difficult to judge whether the EDID data of analog specification or the EDID data of digital specification should be transmitted in response to the transmission request of the EDID data from the computer.
As a method to solve the above problem, a user makes a selection in advance on whether the display monitor should be used as analog specification or digital specification. Then, as a display monitor having the DVI-I interface, one having a switcher which allows the user to determine whether the display monitor should be used as analog specification or digital specification has been conventionally proposed.
When the display monitor is used as analog specification, for example, the input channel of the interface and the EDID data to be transmitted through the DDC communication should be set for analog specification. On the other hand, when the display monitor is used as digital specification, the input channel of the interface and the EDID data to be transmitted through the DDC communication should be set for digital specification.
Further, when adjustment items of OSD (On Screen Display) are different between the analog specification and the digital specification (for example, the OSD of the display monitor of analog specification needs clock-phase adjustment items while the OSD of the display monitor of digital specification does not need), it is necessary to switch the OSD in accordance with the difference.
FIG. 13 is a view showing the switching of EDID data by a switcher of a display monitor of DVI-I connector specification in the background art. As shown in FIG. 13, the switcher is so provided on a DDC line as to establish an access to either EDID data of analog specification or EDID data of digital specification. The user makes a setting on whether the display monitor is used as analog specification or digital specification with the OSD or the like. For example, when the user selects the analog channel, the switcher so operates as to switch the line for an access to the side of the EDID data of analog specification and the when the user selects the digital channel, the switcher so operates as to switch the line for an access to the side of the EDID data of digital specification.
Further, though not shown, the switching of the input channel and the OSD is made in response to the switching by the switcher.
In the above method where the setting on whether to use the analog channel or the digital channel is made in advance, however, when the user makes a wrong setting or the computer connected to the display monitor or the setting of the computer is changed, for example, there is a possibility where analog signals are inputted though the display monitor is set for digital channel or digital signals are inputted though the display monitor is set for analog channel. Since the background-art display monitor can not accommodate such a case, there arises a problem that a correct display is not achieved due to mismatch of support timing or the like.
Further, among the computers capable of outputting digital video signals are ones which stop outputting of the digital video signals when the EDID data sent from the display monitor is not of digital specification, and when the display monitor in which the analog channel is selected by the method shown in FIG. 13 is connected to such computers, there arises a problem that the display monitor receives no video signal and therefore displays nothing.
Furthermore, in the display monitor, it is necessary to assume that the inputted video signals include ones of analog-only type such as D-sub connector input, ones of digital-only type such as DVI-D connector input and ones of mixed digital-and-analog type such as DVI-I connector input, depending on the output mode of the computer. That is because, for example, the computer which sends the video signals does not necessarily adopt DVI-I connector output even if the display monitor adopts the DVI-I connector input. For example, if the user uses a conversion cable or a conversion connector, it is possible to connect, e.g., the display monitor with DVI-I connector input to a computer with other output modes such as D-Sub connector output.